In an attempt to overcome some of the problems associated with traditional chemical and biological waste treatment systems, recent research has focused on the environmental applications of pure enzymes that have been isolated from their parent organisms.
Enzymatic systems fall between the two traditional categories of chemical and biological processes, since they involve chemical reactions based on the action of biological catalysts. Specifically, enzymes are biological catalysts that regulate the multitude of chemical reactions that occur in a living cell, whether it be plant, animal or microbial. They carry out such cellular processes as energy conversion, food digestion and biosynthesis. Enzymes that have been isolated from their parent organisms are often preferred over intact organisms containing the enzyme because the isolated enzymes act with greater specificity, their activity can be better standardized, they are easier to handle and store, and enzyme concentration is not dependent on bacterial growth rates.
Due to their high specificity to individual species or classes of compounds, enzymatic processes can be developed to specifically target selected compounds that are detrimental to the environment. Compounds that are candidates for this type of treatment are usually those that cannot be treated effectively or reliably using traditional techniques. Alternatively, enzymatic treatment can be used as a pretreatment step to remove one or more compounds that can interfere with subsequent downstream treatment processes. For example, if inhibitory or toxic compounds can be removed selectively, the bulk of the organic material could be treated biologically, thereby minimizing the cost of treatment. Due to the susceptibility of enzymes to inactivation by the presence of other chemicals, it is likely that enzymatic treatment will be most effective in those streams that have the highest concentration of the target contaminant and the lowest concentration of other contaminants that may tend to interfere with enzymatic treatment. It is suggested that the following situations are those where the use of enzymes might be most beneficial:
- removal of specific chemicals from a complex industrial waste mixture prior to on-site or off-site biological treatment;
- removal of specific chemicals from dilute mixtures, for which conventional mixed culture biological treatment might not be feasible;
- polishing of a treated wastewater or groundwater to meet limitations on specific pollutants or to meet whole effluent toxicity criteria;
- treatment of wastes generated infrequently or in isolated locations, including spill sites and abandoned waste disposal sites;
- treatment of low-volume, high-concentration wastewater at the point of generation in a manufacturing facility to permit reuse of the treated process wastewaters, to facilitate recovery of soluble products, or to remove pollutants known to cause problems downstream when mixed with other wastes from the plant.
Some potential applications of enzymes that have been identified for the improvement of waste quality include the transformation of aromatic compounds, cyanide, colour-causing compounds, pesticides, surfactants and heavy metals.
Before the full potential of enzymes may be realized, a number of significant issues remain to be addressed. These include: development of low-cost sources of enzymes in quantities that are required at the industrial scale; demonstration of the feasibility of utilizing the enzymes efficiently under the conditions encountered during wastewater treatment; characterization of reaction products and assessment of their impact on downstream processes or on the environment into which they are released; and identification of methods for the disposal of solid residues, among others. Current research is focusing on addressing these issues, especially on the development of enzymatic treatment systems that target the removal of aromatic compounds from wastewaters. Much of our recent efforts have particularly focussed on the use of enzymes to target the transformation of compounds in wastewaters that are present in trace concentrations. These include natural and synthetic estrogens and pharmaceuticals.
Examples of graduate theses
Soegiaman, Selvia Kurniawati (2006) Kinetics of the laccase-catalysed oxidation of aqueous phenol. PhD thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Khan, Usman (2006) Peroxidase-catalyzed oxidation of natural and synthetic gonadal estrogens. MEng thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Wagner, Monika (2001). Enzyme catalyzed polymerization and precipitation of phenols from industrial wastewaters. PhD thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Massam, Alexandra (1999). A kinetic model for the transformation of phenol by peroxidase. MEng thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Ikehata, Keisuke (1998). Characterisation of tyrosinase for the treatment of aqueous phenols. MEng thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Ghioureliotis, Mike (1997). Assessment of residual by-products from the enzyme-catalyzed polymerization of aqueous phenolic compounds. MSc thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Kinsley, Chris (1997). Soybean peroxidase-catalysed treatment of phenol in the presence of polyethylene glycol. MEng thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Buchanan, Ian (1996). Kinetic modelling of horseradish peroxidase catalyzed phenol removal for reactor development. PhD thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.
Wright, Harold (1995). Characterization of soybean peroxidase for the treatment of phenolic wastewaters. MSc thesis, Dept. of Civil Engineering and Applied Mechanics, McGill University.